The silver bleach solutions most commonly used for silver halide photographic elements use ferric complexes to oxidize silver metal to silver halide. It is environmentally desirable to reduce the concentrations and absolute amounts of iron and chelating agents discharged from processing machines, but simply reducing the iron and chelate concentrations results in unacceptable bleach performance. Persulfate bleaches are an alternative to iron-based bleaches, but they are slow acting unless used with bleach accelerators. Most of the commonly used accelerators are low molecular weight thiols which often have undesirable odors and are unstable if incorporated directly into the persulfate bleach.
German Patent Application DE 39 19 551 A1 describes certain persulfate bleaches incorporating a ferric salt, a chelating agent which may be an aminocarboxylic acid, a hydroxycarboxylic acid or a hydroxylpolycarboxylic acid, and a chloride rehalogenating agent. These formulations, however, slowly and incompletely bleach photographic elements with substantial contents of silver bromide and silver iodide. Another disadvantage of these bleaches is that they exhibit the best bleaching performance at low pH values (pH&lt;3), where persulfate suffers acid-catalyzed decomposition. This results in poor stability of the bleaches.
Japanese Kokai No. J5 0026-542 describes a bleaching solution containing an iron chelate and a 2-carboxypyridine. Japanese Kokai No. J5 1007-930 describes a process wherein either the bleach, the fix, or the wash can contain a pyridine-2,6-dicarboxylic acid. Japanese Kokai No. J5 3048-527 describes a bleach containing an aminopolycarboxylic acid metal complex salt and/or a pyridine-2,6-dicarboxylic acid salt. European Patent Application 0 329 088 describes a bleach containing, as one of numerous possible buffers, picolinic acid. None of the above references describe the use of a peracid bleach.
It is desirable to provide a peracid bleaching solution with low metal and ligand concentrations that rapidly and completely bleaches silver halide photographic elements containing a wide variety of silver halide compositions. It is further desirable to provide a ferric-catalyzed persulfate bleach exhibiting excellent silver bleaching at pH values greater than 3, where acid-catalyzed decomposition of persulfate is negligible.